Variable capacity natural gas compressor

ABSTRACT

The invention relates to a method and apparatus for gas compression. More particularly the invention is directed to a variable capacity screw compressor. The screw compressor has an engine, an intake slide valve, and a programmable logic controller for controlling the transmission speed of the engine and for controlling the position of the slide valve. The compressor also has means for monitoring the engine load. The transmission speed and the slide valve position are adjusted in accordance with the engine load. The invention attempts to match available horsepower of the engine with available gas volume by adjusting the compressor to match gas throughput with horsepower. This automated process assists the user in shipping greater volumes of gas.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for gascompression.

BACKGROUND

In the production of natural gas, flow from the well may varysubstantially over the course of time, for various reasons. At the sametime, downstream pressures into which this gas must travel to reach itsmarket may also vary. It is often necessary to place compression at ornear the well site in order to generate both sufficient suctiondifferential on the well to enhance gas production, as well as toprovide sufficient energy to enable the gas to enter downstream gaspipelines. These compressors may be required at the well site, inbooster applications where the production of several or many wells willbe joined, or in fixed gas plant or hydrocarbon processing plants.

There are a number of compression technologies in use in gas productiontoday which all serve specific production needs. Positive displacementcompressors such as screw/reciprocating and vane type compressors arecommon. The production capacity of these compressors are criticallydependent on three key variables: suction pressure, operating speed anddischarge pressure.

Suction pressure is generally a function of the performance of the wellor header in question, while discharge pressure is generally dependenton the line pack or flowing pressure of sales gas pipelines into whichthe gas is delivered. Compressor speed is the only practical tool forvariation in production for positive displacement compressiontechnologies.

Of the common positive displacement compressors, the screw compressor isa preferred technology primarily because it is cost effective. The screwcompressor is so named as it consists of two screw shaped rotors whosethreads mesh during rotation along their length to highly engineeredtolerances, forming a compression “chamber” as the clearance between therotors at the beginning of the thread declines to the end of the thread.A screw compressor traps a fixed volume of gas on the suction side andincreases its pressure by reducing the internal volume of thecompression chamber, thereby raising its pressure at the discharge side.

Conventionally, a screw compressor is directly coupled to an enginewhich may be powered by natural gas, and which may produce 30 to over1,000 horsepower. Placed at the well site or on a header pad receivinggas from a number of wells, these units run at a fixed speed—offering afixed throughput volume based on the suction pressure available. Typicalinstallations run at the engine speed of 1,800 rpm, although there aregeared units available capable of taking advantage of the compressorscapacity for higher speeds and thus higher volumes. These gear unitsmust be designed for the expected suction/discharge pressures and volumeinputs for a given application, and suffer significant degradation inefficiency when conditions change.

The key problem of the standard natural gas drive design is that adirect-coupled machine is incapable of matching the best compressorspeed with available horsepower to maximize throughput given the currentsuction and discharge pressure. The compressor cannot turn at speedsgreater than engine speed, unless a speed-multiplying gearbox is used.The gearbox then restricts the compressor speed to the gear ratio of theengine speed. Natural gas engines generally rotate at 1,800 rpm,subsequently restricting the compressor to this speed or multiples ofthis speed. As positive displacement machines, compressors take a fixed“gulp” of natural gas with each rotation. Obviously, increasing thenumber of rotations provides more throughput, resulting in greater gassales.

When the critical conditions begin to change, then the fixed speed modelbecomes increasingly inefficient. If suction pressure drops, therebyfreeing horsepower demand on a fixed setup, this horsepower cannot beutilized, as the compressor continues to operate at the same speed.Conversely, if the discharge pressure climbs unexpectedly, the enginewill run out of horsepower and the unit will shut down on high dischargepressure.

A screw compressor may be manually adjusted to match current conditions.The process generally requires human intervention to adjust thethroughput. When production conditions change through well depletion, ormore importantly, new drilling or current well optimization, thestandard design requires human intervention to either resize thecompressor, the engine or the gearbox to rematch hardware withthroughput. Such intervention is expensive, time-consuming andinconvenient.

Therefore, there is a need in the art for a variable output enginedriven screw compressors which are not currently known to those skilledin the art.

SUMMARY OF THE INVENTION

In one aspect, the invention comprises a variable capacity screwcompressor comprising:

-   -   (a) an engine coupled to a variable speed hydrostatic        transmission;    -   (b) an intake slide valve, and actuation means for varying the        intake slide valve;    -   (c) means for monitoring engine load;    -   (d) means for varying transmission speed;    -   (e) control means comprising an engine load sensor input        operatively connected to the engine load monitoring means, a        transmission speed output operatively connected to the        transmission speed varying means, an intake slide valve output        operatively connected to the intake slide valve actuation means,        wherein said control means automatically adjusts transmission        speed, and intake slide valve position, according to engine        load.

The compressor may further comprise a Vi control slide valve, andactuation means for varying the Vi control slide valve operativelyconnected to the control means, which automatically adjusts Vi controlslide valve position according to engine load.

In one embodiment, the variable speed hydrostatic transmission comprisesa variable speed motor, and a variable speed pump. The control means maybe operatively connected to a speed control device on the variable speedmotor, or to a speed control device on the variable speed pump, or speedcontrol devices on both the motor and the pump.

In one embodiment, the compressor may further comprise a gas recycleline connecting a gas discharge end of the compressor to a gas suctionend, a recycle valve controlling flow through the gas recycle line, andmeans for actuating the recycle valve, said actuation means operativelyconnected to the control means.

In one embodiment, the compressor may comprise a PID loop operativelyconnected to a suction pressure control valve and a suction pressuretransmitter, wherein said PID loop operates independently of the controlsystem. Furthermore, the compressor may also comprise a PID loopoperatively connected to a discharge pressure control valve and adischarge pressure transmitter, wherein said PID loop operatesindependently of the control system.

In another aspect of the invention, the invention may comprise a methodof efficiently compressing gas from a gas well using a variable capacitycompressor having an intake slide valve, and driven by an engine coupledto a hydraulic transmission including a variable speed pump and variablespeed motor, said method comprising the steps of:

-   -   (a) sensing engine load;    -   (b) attempting to maintain engine load within a desired range        by:        -   i. adjusting compressor speed by varying the hydraulic            transmission in response to changes in engine load; or        -   ii. adjusting the intake slide valve in response to changes            in engine load; or        -   iii. both.

In one embodiment, the intake slide valve is first adjusted, and thencompressor speed is adjusted. If engine load is not brought within thedesired range by adjusting compressor speed and intake slide valve, thenthe Vi control slide valve may also be adjusted in order to vary engineload.

Suction pressure may be controlled independently of controlling engineload, and as well, discharge pressure may also be controlledindependently of controlling engine load.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of an exemplary embodimentwith reference to the accompanying simplified, diagrammatic,not-to-scale drawings. In the drawings:

FIG. 1 is a schematic representation of one embodiment of the presentinvention.

FIG. 2 is a schematic representation of one embodiment of a screwcompressor.

FIG. 3 is a schematic representation of one embodiment of a hydraulicpump and motor.

FIG. 4 is a schematic representation of one embodiment of an engine.

FIG. 5 is a schematic representation of one embodiment of a PLC.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides for a variable output gas compressor.When describing the present invention, all terms not defined herein havetheir common art-recognized meanings. To the extent that the followingdescription is of a specific embodiment or a particular use of theinvention, it is intended to be illustrative only, and not limiting ofthe claimed invention. The following description is intended to coverall alternatives, modifications and equivalents that are included in thespirit and scope of the invention, as defined in the appended claims.

Production flow from a conventional natural gas well is typically notconstant, and as a result, the suction pressure of a gas compressorpackage constantly varies. With a constantly varying suction pressure, acompressor package is subjected to high recycle rates in order tomaintain the suction throughput. This leads to wasted horsepower, fuelgas and increased heat loads. For example, in a well where a plungerlift is installed as an optimization technique to unload unwanted water,gas production and thus suction pressure can vary significantly. Whileplunger lift device design is varied, a typical design consists of ametal cylinder which rises and falls in the production tubing in thewell, thereby carrying water out of the well bore. During the plungerdown cycle, gas production will cease from the well. During an up cycle,gas flow will resume.

The use of a plunger lift pose significant problems for well sitecompression. The varying suction pressure and volume conditions do notallow efficient compressor operation. A conventional compressor must goto high recycle rates during periods where gas flow ceases, which leadsto overheating problems, and eventual shutdown. It is possible tomanually unload the compressor, but requires human intervention. As someplungers might cycle a dozen or more times a day, this approach issimply impractical.

The present invention may compensate for these changes by making theaforementioned control changes and ensure driving horsepower isefficiently utilized, without the attendant problems of humanintervention or interrupted operation of the equipment.

This invention comprises a variable output screw compressor packagedesigned to maximize the amount of gas that can be moved to sales as theoperating conditions change. The screw compressor comprises a controlsystem which is designed to utilize more horsepower available from theengine when it is advantageous to do so, by varying one or more of thecompressor speed, suction pressure, discharge pressure, and slide valveposition in a dynamically controlled environment.

A capacity control or intake slide valve is available on most screwcompressors, and is effectively a device which can vary the volume ofgas permitted to enter the compression chamber with each rotation. Inaddition, a screw compressor may have a volume ratio control slidevalve, which varies the volume index, or Vi, of the compressor. Thevolume index is the ratio of the volume of trapped gas at the start ofcompression, versus the volume of gas as it is discharged. Some slidecontrol valves are equipped to decrease discharge port area and allowsome discharge gas to shunt back to the suction side. This approachallows a single slide valve to achieve both capacity control and volumeratio control.

In operation, the present invention attempts to continuously matchavailable horsepower with available gas volume by adjusting thecompressor to match gas throughput with horsepower. This process isautomated, requires no human intervention, and assists the producer toship a greater volume of gas to market, given the physical limitationsof the equipment available.

This adjustment and control process comprises the use of variable speedhydraulic coupling devices, as well as hydrodynamic, electrical andinstrument controls to adjust one or more of engine speed, motor speed,pump output, compressor speed, discharge pressure, and slide valveposition to match gas pressure conditions and available driverhorsepower.

In one aspect, as shown in the general schematic of FIG. 1, theinvention comprises a variable output compressor comprising a gascompressor (10), an engine (12), a transmission connected between thecompressor and the engine, and a control system (14) for controlling theoutput of the compressor in response to variable conditions. In oneembodiment, the transmission is a hydrostatic transmission involving ahydraulic pump (16) operatively connected to a hydraulic motor (18),where the hydraulic pump is driven by the engine, and the hydraulicmotor drives the compressor. In one embodiment, one or both of thehydraulic pump and motor is a variable displacement hydraulic pump ormotor. Variable displacement hydraulic pumps and motors are well knownin the industry and need not be further described here.

In another aspect, the invention comprises a method of maximizingcompressor efficiency by hydrodynamic coupling of the engine to thecompressor and using a control algorithm linked to electro-mechanicalsystems to control the hydrodynamic coupling, and various otherparameters, to optimize the speed of the compressor. These controls mayregulate the inlet volume of gas to the compression chamber in thecompressor, by controlling the capacity control slide valve and as wellmay regulate the compression ratio in the compression chamber, bycontrolling the volume ratio control slide valve. Hydraulic motor speed,as well as hydraulic pump fluid volumes may also be controlled tomaximize compressor efficiency. The control system monitors engine loadand activates system components to relieve excess engine load, or toincrease engine load, where appropriate. In one embodiment, compressorcapacity may be primarily controlled by two methods: compressor speedand slide valve position, either or both capacity control and volumeratio control.

As shown in FIG. 2, a gas compressor (10) has a gas inlet (1) and a gasdischarge (2). The capacity control slide valve (3) is actuated by aslide valve load solenoid (SY100) and a slide valve unload solenoid(SY101). The slide valve position is reported by sensor (RPT120).Suction gas pressure is reported upstream from the compressor bypressure transmitter (PT100) and is controlled by a suction pressurecontrol valve (4), which is actuated by a suction pressure control valvecontroller (PY100). Discharge pressure is reported by pressuretransmitter (PT140), and is controlled by discharge pressure controlvalve (5), which is actuated by a discharge pressure control valvecontroller (PY140).

In one embodiment, a variable speed hydraulic motor (18) is driven by ahydraulic pump (16) which itself may be a variable speed unit. The motor(18) is stroked by a hydraulic motor control (PY500), while the pump maybe varied by hydraulic pump control (PY600).

The engine (12) may be a natural gas engine, as is well known in theart. Speed of the engine, for display purposes or control purposes, maybe reported by a speed transmitter (ST400). Intake manifold pressure,which is representative of engine load, may be reported by pressuretransmitter (PT400). An engine fuel shutoff solenoid (FY410) may beprovided to cut off fuel to the engine in order to effect a shutdown.

A control system (14) of the present invention reads the data inputs andactuates the control devices described herein. The control system maycomprise a programmable logic controller or PLC. A PLC is a computertypically used for automation of industrial process, and may runsoftware stored in memory. The controller may comprise a microprocessoror a microcontroller with on-chip resources, such as an A/D converter,ROM (EPROM), RAM. The microprocessor or microcontroller is suitablyprogrammed, for example in software or firmware, to perform theoperations described below as will be within the understanding of thoseskilled in the art.

In one embodiment, the electro-mechanical controllers are driven from apressure sensor PT400 located in the engine inlet manifold which gives adirect indicator of engine load by measuring manifold pressure. Innormally aspirated engines, as load increases manifold pressuredeclines. In turbocharged engines, as load (HP demand) increases,manifold pressure increases. The control system will monitor engine loadand may adjust one or more of compressor speed, the capacity controlslide valve; or the Vi control slide valve, in an effort to stabilizethe manifold pressure at a preset level, thus engaging as much availablehorsepower as possible to the compression of gas. Speed range for thecompressor may be in the 1,500 rpm to 5,000 rpm range. Speed adjustmentsmay be accomplished by repositioning the swashplate in the hydraulicpump (16) or the hydraulic motor (18), or both, through control signalsfrom the control system (14).

Thus, in one embodiment, the first response to a change in engine loadis to adjust compressor speed by varying the hydraulic motor (18). Ifspeed control is insufficient to relieve excessive load on the engine,then the capacity control intake slide valve (22) is repositioned toreduce the inlet gas volume into the compressor, thereby reducing load.In addition, or alternatively, a Vi control slide, if so equipped, maybe repositioned to reduce the volume index of the compressor, therebyreducing load. Should this control adjustment prove insufficient toreach optimal load, then the hydraulic pump (16) will begin to reduceits fluid contribution to the hydraulic motor (18).

In one embodiment, a compressor may have a gas recycle line (6) andcontrol valve (7) actuated by controller PY150, which provides anothermeans of controlling compressor capacity. Opening the recycle controlvalve (7) will have the effect of reducing load on the engine.

The controls are capable of a continuous and unattended adjustment ofthe package, and its controls entirely dependent on compressor loadconditions. In booster applications, where the number of wells producinginto a given compressor suction might vary significantly over a numberof hours or days, conventional prior art screw compressor packagesrequire constant human intervention in order to ensure the compressor isloaded correctly.

An important advantage of this design is on the discharge side ofcompressor performance. On conventional screw compressor packages, thefixed speed permits a very limited window of discharge operation. Whenthe compressor is configured for a given suction/discharge regime, anyvariation is this regime leads, at best, to operating problems withhuman intervention and, at worst, to equipment shutdown, and lost gassales.

In addition to the primary compressor speed control, and control of thesuction slide valve and Vi control position, it is possible toseparately control both suction pressure and discharge pressure. Thus,in one embodiment, as shown in FIG. 2, the compressor suction pressureis controlled by the control system (14) via a PID loop with anadjustable setpoint. Suction gas pressure is reported upstream from thecompressor by pressure transmitter (PT100) and is controlled by asuction pressure control valve (4), which is actuated by a suctionpressure control valve controller (PY100). As the suction pressure fallsbelow a desired setpoint or range, the control valve (4) will open. Asthe suction pressure rises above the setpoint or range, the controlvalve (4) will close.

As shown in FIG. 2, the compressor discharge pressure is controlled viaPID loop with an adjustable setpoint. Discharge pressure is reported bypressure transmitter (PT140), and is controlled by discharge pressurecontrol valve (5), which is actuated by a discharge pressure controlvalve controller (PY140). As the discharge pressure falls below thesetpoint or range, the control valve (5) will throttle closed. As thedischarge pressure rises above the setpoint, the control valve (5) willthrottle open.

In one embodiment, in operation and upon startup, the control system itwill try to achieve full engine load by first loading the compressor,after the initial warm-up period. Compressor load may be achieved byactuating the intake slide valve and increasing the compressor speed, asdescribed above. When full engine load is achieved, then the system willstop loading the compressor. If the intake slide valve is at 100% andcompressor speed is at 100%, and the engine is still not fully loaded,then the Vi control slide valve may be adjusted to increase Vi.

Conversely, if there is a need to reduce engine load, such as when theengine load begins to increase due to increased discharge pressure, thecontrol system will first drop compressor speed until engine loadstabilizes or a minimum desired compressor speed is reached. Ifcompressor speed is dropped to a minimum, and engine load is stillincreasing, then the intake slide valve, or the Vi control slide valve,or both, will begin to unload until engine load reaches the desiredlevel.

Intake suction control pressure may be separately monitored andcontrolled. If suction pressure is lower than the desired setpoint, thenthe suction pressure control valve may open until suction pressurestabilizes. If suction pressure continues to fall after the valve is at100% open, the compressor speed will begin to drop. If the suctionpressure continues to drop after achieving minimum speed on thecompressor, the intake slide valve will begin to unload. As well, therecycle valve may throttle open to maintain minimum suction pressure ifminimum suction pressure cannot be maintained by slowing compressorspeed and unloading the intake slide valve. preferably after a timeddelay, in order to reduce the engine load and save fuel. In oneembodiment, the opening of the recycle valve may be delayed using atimer. Also, if all other controls have been implemented, i.e. with theunit running 0% on the slide valve, minimum compressor speed, and therecycle valve open to maintain suction pressure, engine speed may alsobe reduced, preferably after another timed delay. For example, theengine may kick down to 1000 rpm from the normal speed of 1800 rpm.

The recycle valve (7) may also throttle open if the discharge pressureapproaches the maximum allowable working pressure (MAWP) of the unit.

In one embodiment, the control system is operatively connected to theengine, allowing control over engine parameters such as ignition timing,air/fuel ratio and engine speed. Thus, the control system may handle allshutdowns as well as the starting of the engine. In one embodiment,speed feedback to the PLC will be for display purposes only, with theexception of the start-up sequence. The gas starter will disengage oncethe minimum start speed has been exceeded.

Engine shutdown will be controlled by the control system by killing theignition, as well as de-energizing the fuel supply solenoid.

Engine speed may be controlled with the use of a solenoid incorporatedinto a throttle linkage. Conventional means to sense engine speed may beused such as a magnetic pick-up on the flywheel and the use of a signalconverter to provide an input signal (ST400) to the control system (14).

Shutdown Summary

There are many scenarios where a compressor shutdown is necessary ordesirable, examples of which follow. In each case, the control systemwill react to an input from a sensor and initiate compressor shutdown.

1) Low Suction Pressure

2) High Suction Pressure

3) Low Lube Oil Level

4) High Lube Oil Level

5) High Discharge Temp

6) High Vibration

7) Suction Scrubber High Level

8) High Discharge pressure

There are other scenarios where engine shutdown is necessary ordesirable. In these cases, the control system will react to a sensorinput, and initiate engine shutdown/

1) Low Oil Pressure

2) High Coolant Temp

3) Overspeed

4) High Vibration

5) High Manifold Pressure

Start-Up Sequence

Prior to starting the unit, the operator should ensure that all fluidlevels are within their normal ranges, all valves are in their operatingpositions, and all necessary safety devices are in place and active.

If the unit was shutdown due to a fault condition or shutdown, theoperator should ensure that the fault condition has been repaired orremoved. The alarm will have to be acknowledged on the control panel,and then reset prior to start up.

The sequence of events in a start-up is as follows:

-   -   a) Engine is started and the control system checks to ensure the        compressor is unloaded with the intake slide valve at 0%.    -   b) Engine runs up to operating speed, such as 1800 rpm.    -   c) Once the engine reaches operating temperature, the compressor        is cleared to start and the hydraulic pump begins to stroke and        the compressor begins to rotate. The pump continues to stroke        until it reaches 100%.    -   d) When the pump is at full stroke, the control system will        begin to move the intake slide valve and load the compressor.    -   e) Once the compressor reaches full load (slide valve to 100%),        the control system will begin to stroke the hydraulic motor to        increase the compressor speed towards 100%.

Concurrently, the suction valve controller will be active, trying tomaintain its setpoint. As the compressor begins to rotate and move gas,suction pressure will decrease, and the valve will begin to throttleopen to maintain its setpoint. Also, the discharge valve controller willbe active, although it will not begin to open until the dischargepressure upstream of the discharge valve reaches the controllersetpoint. The discharge pressure controller setpoint is typically set tomaintain the minimum pressure required by the internal lubricationsystem of the compressor to function properly.

Shutdown Sequence

A shutdown can be initiated by either the operator, or by one of theprotective shutdown devices on the unit. Sequence is as follows for anoperator shutdown:

-   -   a) Operator initiates stop command via control panel.    -   b) The control system will drop the signals to the hydraulic        motor and to the hydraulic pump effectively stopping the        compressor.    -   c) The controls will then fully unload the compressor (slide        valve to 0%).    -   d) The engine will throttle down to minimum speed, (approx 1000        Rpm), until the operator shuts the engine off via the control        panel.

In the event of a protective shutdown, the control system will close thefuel supply to the engine, effectively stopping the entire unit almostimmediately.

In the event of an Emergency Shut Down, (ESD), the controls will closethe fuel supply to the engine as well as grounding out the ignitionsystem to ensure the engine/compressor stops immediately.

In an example of field use of one embodiment of the present invention,the discharge pressure for normal operation in a booster application was225 psi discharge into a sales gas line. This line led to the suction ofa large reciprocating compressor owned by the gas utility. When thedownstream reciprocating compressor breaks down, then line pressure inthe sales line will increase. All of the producer's regular boostercompressors automatically shut down when the line pressure reached 275psi, as this load exceeded the available horsepower for these units. Avariable output compressor of the present invention may, unattended,adjust the rotational speed and load on the compressor downward toensure there is sufficient horsepower available to keep gas moving intothe sales line—even at discharge pressures above 325 psi. Thiscompressor was able to support operations throughout the day and nightuntil the downstream reciprocating compressor was repaired. This enabledthe producer to continue producing sales gas and generating revenue,where other compressors were required to shut down.

The preceding detailed description of specific embodiments of thepresent invention does not limit the implementation of the invention toany particular programming language or signal processing architecture.In one embodiment, the present invention is implemented, at leastpartly, using a digital signal processor operating under stored programcontrol. It will be understood that the present invention may beimplemented using other architectures, including a microprocessor, amicrocontroller, a field programmable logic device such as a fieldprogrammable gate array, discrete electronic and logic components orcombinations thereof. Any limitations described herein as a result of aparticular type of architecture or programming language are not intendedas limitations of the present invention.

1. A variable capacity screw compressor comprising: (a) an enginecoupled to a variable speed hydrostatic transmission; (b) an intakeslide valve, and actuation means for varying the intake slide valve; (c)means for monitoring engine load; (d) means for varying transmissionspeed; and (e) control means comprising an engine load sensor inputoperatively connected to the engine load monitoring means, atransmission speed output operatively connected to the transmissionspeed varying means, an intake slide valve output operatively connectedto the intake slide valve actuation means, wherein said control meansautomatically adjusts transmission speed, and intake slide valveposition, according to engine load.
 2. The compressor of claim 1 furthercomprising a Vi control slide valve, and actuation means for varying theVi control slide valve operatively connected to the control means, whichautomatically adjusts Vi control slide valve position according toengine load.
 3. The compressor of claim 1 wherein the variable speedhydrostatic transmission comprises a variable speed motor, and avariable speed pump.
 4. The compressor of claim 1 wherein the controlmeans is operatively connected to a speed control device on the variablespeed motor, or to a speed control device on the variable speed pump, orspeed control devices on both the motor and the pump.
 5. The compressorof claim 1 wherein the control means comprises a programmable logiccontroller.
 6. The compressor of claim 1 wherein the control meansresponds to a change in engine load during operation by first adjustingcompressor speed, then by adjusting intake slide valve position, andlastly by adjusting Vi control slide valve position.
 7. The compressorof claim 6 further comprising a gas recycle line connecting a gasdischarge end of the compressor to a gas suction end, a recycle valvecontrolling flow through the gas recycle line, and means for actuatingthe recycle valve, said actuation means operatively connected to thecontrol means.
 8. The compressor of claim 1 further comprising a suctionpressure control system comprising a PID loop operatively connected to asuction pressure control valve and a suction pressure transmitter,wherein said PID loop operates independently of the control system. 9.The compressor of claim 8 further comprising a discharge pressurecontrol system comprising a PID loop operatively connected to adischarge pressure control valve and a discharge pressure transmitter,wherein said PID loop operates independently of the control system. 10.A method of efficiently compressing gas from a gas well using a variablecapacity compressor having an intake slide valve, and driven by anengine coupled to a hydraulic transmission including a variable speedpump and variable speed motor, said method comprising the steps of: (a)sensing engine load; (b) attempting to maintain engine load within adesired range by: i. adjusting compressor speed by varying the hydraulictransmission in response to changes in engine load; or ii. adjusting theintake slide valve; or iii. both.
 11. The method of claim 10 whereincompressor speed is first adjusted, and then intake slide valve isadjusted, if engine load is not brought within the desired range byadjusting compressor speed.
 12. The method of claim 11 wherein thecompressor further comprises a Vi control slide valve, and the Vicontrol slide valve is adjusted in order to vary engine load.
 13. Themethod of claim 10 further comprising the step of controlling suctionpressure independently of controlling engine load.
 14. The method ofclaim 13 further comprising the step of controlling discharge pressureindependently of controlling engine load.